Method for forming high efficiency geothermal wellbores

ABSTRACT

Wellbore synthesis techniques are disclosed suitable for use in geothermal applications. Embodiments are provided where open hole drilled wellbores are sealed while drilling to form an impervious layer at the wellbore/formation interface. The techniques may be chemical, thermal, mechanical, biological and are fully intended to irreversibly damage the formation in terms of the permeability thereof. With the permeability negated, the wellbore may be used to create a closed loop surface to surface geothermal well operable in the absence of well casing for maximizing thermal transfer to a circulating working fluid. Formulations for the working and drilling fluids are disclosed.

FIELD OF THE INVENTION

The present invention relates to geothermal wellbore creation and moreparticularly, the present invention relates to methods for modifying thepermeability of a given formation for creating high efficiencygeothermal wellbores with improved thermal and mechanicalcharacteristics additionally with working fluid formulations.

BACKGROUND ART

Geothermal energy recovery is an attractive method of capturing energyand has obvious environmental appeal considering the renewabilityaspect.

The prior art has focused on numerous issues in respect of permeability,well geometries, working fluids, multilateral well configuration andpower production. Examples of attempts to ameliorate these issues willbe discussed in turn.

Initially, in respect of formation damage, Badalyan et al. in_LaboratoryStudy on Formation Damage in Geothermal Reservoirs Due to FinesMigration, Proceedings World Geothermal Congress 2015 Melbourne,Australia, 19-25 Apr. 2015, teach:

-   “Here we present a new method to assess formation damage in    geothermal reservoirs. It is long known that formation damage is    caused by mobilisation, migration and straining of natural reservoir    fines . . . . Velocity-induced fines migration is responsible for a    non-significant reduction of rock permeability leading to initial    formation damage. Following low-ionic strength water injection    increases electrostatic repulsion force between clay particles and    sand surface, further mobilizes particle resulting in formation    damage. Mobilised fines with mixed-layer illite/chlorite mineralogy    are responsible for rock permeability reduction due to pore-throats    clogging.”-   Fines migration is one of the most widely spread physics mechanisms    of formation damage in oil and gas wells. Numerous recent    publications report well impairment by fines migration in geothermal    fields. [Emphasis mine]

In Mechanisms of Formation Damage in Matrix Permeability GeothermalWells Conference: International Geothermal Drilling and CompletionsTechnology Conference, Albuquerque, N.M., USA, 21 Jan 1981, Bergosh etal. indicate in an abstract of their presentation:

-   “Matrix permeability geothermal formations are subject to damage    during well drilling and completion. Near well bore permeability    impairment that may occur as a result of particulate invasion, and    chemical interaction between formation clays, drilling mud filtrates    and formation brines is investigated. Testing of various filtration    chemistries on the permeability of East Mesa sandstone indicates    that permeability is significantly impaired by the flow of low    salinity formation brines. This damage is attributed to cation    exchange and removal processes which alter the stability of clay    structures. Fluid shearing dislodges particles, which clog pore    throats, irreversibly reducing permeability. The test program    investigating the effects of mud-transported particles on geothermal    formations is still in progress. The rationale, apparatus and test    procedures are described. Final results of this testing will be    presented at the conference.” [Emphasis mine]

Clearly, the loss of permeability in these geothermal environments hassignificant impact on the production of the wellbore and concomitantenergy recovery.

Tchistiakov, in Physico-Chemical Aspects of Clay Migration andInjectivity Decrease of Geothermal Clastic Reservoirs, Proceedings WorldGeothermal Congress 2000, Kyushu-Tohoku, Japan, May 28-Jun. 10, 2000,states in his summary:

-   “The permeability damage potential can be evaluated only via    broad-minded and interdisciplinary thinking, rather than through    automatic application of mathematical equations and laboratory test    results. We are convinced that better understanding of the    fundamental physico-chemical principles of clay particle stability    and transport in porous media will help the reservoir specialists to    develop better techniques and apply more effective existing ones for    preventing in-situ clay induced formation damage of geothermal    reservoirs.”

The paper establishes the clay damage to permeability of the drilledwell.

Barrios et al, at the Short Course on Geothermal Development andGeothermal Wells, organized by UNU-GTP and LaGeo, in Santa Tecla, ElSalvador, March 11-17, 2012, Acid Stimulation of Geothermal Reservoirs.In the presentation, the authors indicate:

-   “Both injection and production wells can be clogged, reducing their    production capacity and injectivity below their existing potential.    The main reasons for these obstructions may be: Invasion of drilling    fluids (mainly bentonite mud) inside the micro fractures of the    reservoir; Entry of rock fragments or cuttings, during the drilling    process while encountering a total loss circulation; Entry of great    amounts of Total Dissolved Solids; Reinjection water with high    silica scaling potential; Formation of fine-grained solids displaced    by clay migration; Entry of amorphous silica fragments from the    reinjection pipelines due to the cooling and heating processes s    after maintenance; Calcite scaling in the perforated liner and/or    production casing. The key to ensure a continuous flow for power    generation is to control all the possible causes of obstruction. It    is a well-known fact that the geothermal industry has been using    similar technology and practices of the oil industry for the last 50    years. Since oil and gas wells show analogies with regards to    scaling problems and mud damage, similar techniques may be applied    to prevent permeability problems in order to improve infectivity and    productivity capacity in geothermal wells. A cost-effective and    widely used solution is the application of acids to dissolve scales    and obstruction produced by solids.”

You et al. in New Laboratory Method to Assess Formation Damage inGeothermal Wells, SPE European Formation Damage Conference andExhibition, 3-5 June, Budapest, Hungary 2015 presented a paper, theabstract of which states:

-   “The new method to assess permeability damage in geothermal    reservoirs and predict well productivity decline is presented. The    laboratory methodology developed aims to determine permeability    decline from mobilisation, migration and straining of natural    reservoir fines. Laboratory coreflood testing with constant and    stepwise decreasing ionic strength has been performed with    measurements of the pressure drop along the core and accumulated    effluent particle concentration. Stabilisation of rock permeability    occurs after injection of numerous pore volumes, suggesting slow    drift of mobilised particles if compared with the carrier water    velocity. Low ionic strength water increases electrostatic repulsion    forces between clay particles and sand grain surfaces, further    mobilising particles and resulting in formation damage. Kaolinite    and illite/chlorite mixed layer clay minerals are identified by    SEM-EDAX analysis and are the minerals primarily responsible for the    permeability damage. The competitive effects of decreasing water    viscosity and weakening electrostatic attraction on the attached    particle concentration during temperature increase have been    observed. The micro-modeling of the fine particle mechanical    equilibrium shows that the water viscosity effect on the fine    particle attachment dominates. It results in decreased fines    detachment and permeability decline at high temperatures.”

Turning to drilling fluids, numerous advances have been made in theformulations to mitigate wellbore consolidation issues, permeation,sealing inter alia. These are also related to the discussion aboveregarding formation damage.

In U.S. Pat. No. 6,059,036, issued May 9, 2000, Chatterji et. al.provide methods and compositions for sealing subterranean zones.Generally, the text indicates:

-   “The present invention provides improved methods and compositions    for sealing subterranean zones and terminating the loss of drilling    fluid, crossflows and/or underground blow-outs. The methods of the    present invention for sealing a subterranean zone basically comprise    the steps of preparing a viscous set delayed sealing composition of    this invention, placing the sealing composition in a subterranean    zone to be sealed and allowing the sealing composition to set into a    rigid sealing mass therein.-   The sealing compositions of this invention are basically comprised    of an aqueous alkali metal silicate solution, a gelling agent for    increasing the viscosity of the solution and a delayed activator for    polymerizing or cross-linking the alkali metal silicate and causing    the sealing composition to set into a rigid sealing mass.-   As mentioned above, in applications involving a need for a low    density sealing composition or where a large cavernous subterranean    zone is encountered which must be sealed, the sealing composition    can be foamed to form an energized and expanding sealing    composition. The non-foamed and foamed compositions can also include    extending and/or bridging agents to facilitate filling and sealing a    zone.”

The document is useful to demonstrate the effectiveness of alkali metalsilicate compositions for fluid loss prevention and general wellboresealing.

Ballard, in U.S. Pat. No. 7,740,068, issued Jun. 22, 2010, disclosessilicate-based wellbore fluid and methods for stabilizing unconsolidatedformations. It is stated in the text that:

-   “Advantageously, embodiments of the present disclosure may provide    for treatment fluids or pills that may be used to stabilize    unconsolidated or weakly consolidated regions of a formation. Using    solid or particulate silicate precipitating agents may allow for a    slower reaction or gelation time between the silicate and the    silicate precipitating agents. A slower reaction time may allow the    gel components, the silicate and the silicate precipitating agent,    to more fully permeate the unconsolidated formation prior to    gelation. Additionally, by providing silicate precipitating agent as    a solid particulate matter on a micron or sub-micron scale, the    silicate precipitating agent may experience less hindrance in    permeating the formation. ”

This document is useful to substantiate that silicate compounds haveutility in stabilizing a formation.

U.S. Pat. No. 8,822,386, issued to Quintero et al., Sep. 2, 2014,provides Nanofluids and methods of use for drilling and completionfluids. This document further adds to the body of work relating todrilling fluids and teaches the usefulness of such fluids duringdrilling. The text provides further detail in this regard.

-   “In one non-limiting example a drilling fluid containing    nanoparticles is expected to be useful to stabilize the wellbore    during drilling, particularly the shale regions encountered during    drilling which may contain areas tend to slough into the borehole or    have clays which undesirably swell when contacted with water    introduced as part of the drilling fluid. Such a drilling fluid may    be an aqueous-based fluid such as a WBM, a non-aqueous based fluid    such as an OBM or SBM, or a combination thereof, namely an emulsion.    A surfactant may be present in an amount effective to suspend the    nanoparticles in the fluid. Nanoparticles expected to be useful in    such shale stabilizing fluids are those which contain    functionalities that associate with the shale and help keep it in    its original condition or as close to its original condition as    possible, that is strengthen the borehole wall. Nanoparticles having    a surface charge may assist with this shale stabilization, such as    carbon nanotubes. Further, the small size of the nanoparticles    permits them excellent access to the shale matrix to inhibit both    the external and internal surfaces of clays to minimize damage to    the structure of the shale. ”

Use of high ratio aqueous alkali silicates in drilling fluids isdisclosed in U.S. Pat. No. 9,212,304, issued to McDonald, Dec. 15,2015.The teachings provide further evidence as to the utility of suchcompositions as used in the oil and gas industry. The documentindicates:

-   “The present invention provides a method for wellbore stabilization    in the drilling of wells for conventional and unconventional energy    sources, these include but are not limited to conventional oil and    gas wells, shale gas and “tar sands”. The method provides for a    drilling fluid that can among other things, reacts with shale to    prevent hydration and dispersion, seal microfractures, prevent shale    delamination, prevent bitumen accretion, allow the drilling of    depleted zones.-   This invention uses larger, more complex polysilicate anions found    in aqueous, high ratio alkali silicates. These high ratio aqueous    silicates are beyond the ratio of traditional, commercially    available silicates. These polysilicate anions facilitates quicker    precipitation and polymerization reactions compared to standard    ratio aqueous silicates. The higher ratio results in a silicate with    lower salinity making for a more environmentally friendly drilling    fluid. High ratio, aqueous alkali silicate can be added to the    drilling fluid at a wide range of concentrations to achieve the    necessary wellbore stabilization. Soluble silica level in the    drilling fluid can range from a25% to about 6% by weight of the    drilling fluid. The pH of the drilling fluid is preferably    maintained above pH 10.”

Stephen Bauer et al, in High Temperature Plug Formation with Silicates,presented at the Thirtieth Workshop on Geothermal Reservoir Engineering,Stanford University, Stanford, Calif., Jan. 31-Feb. 2, 2005, disclose amethod for temporary plugging of specific lost circulation zones, whichare commonly encountered during drilling operations in oil, gas, andgeothermal industries. “This work describes a chemical solution ofexploiting silicates' unique gelling properties in an environmentallyfriendly and cost-effective way to form plugs for use in water shutoffstrategy, steam flooding, and high-temperature grouting/plugging forlost circulation.” The paper does not contemplate formulation andapplication of a silicate-based drilling fluid to seal wellbores andmultilateral junctions to form a closed-loop geothermal system.

Halliburton Energy Services, in PCT filing WO 03/106585, describes amethod for forming chemical casing “A well bore is drilled with adrilling fluid having a pH in the range of from about 6 to about 10 andcomprised of water, a polymeric cationic catalyst capable of acceptingand donating protons which is adsorbed on the unconsolidated clays,shales, sandstone and the like, a water soluble or dispersible polymerwhich is cross-linkable by a thermoset resin and caused the resin to behard and tough when cured and a water soluble or dispersible thermosetresin which cross-links the polymer, is catalysed and cured by thecatalyst and consolidates the weak zones or formations so that sloughingis prevented.”

The document does not contemplate formulation and application of thedrilling fluid to seal wellbores and multilateral junction's to form aclosed-loop geothermal system, nor consider the maintenance of the sealover a typical lifecycle of a geothermal system of 50 years or more.

Another example in the multilateral art is seen in Halliburton EnergyServices, U.S. Pat. No. 9,512,705, which teaches a mechanicalmultilateral wellbore junction to isolate several horizontal wells fromthe surrounding rock. Complex and expensive mechanical or cementedjunctions requiring multiple installation steps are typical in thevolumes of prior art. These multiple installation steps necessitateinterruptions in forward drilling operations such as bringing the drillbit and bottomhole assembly to surface or waiting on cement.

Another drawback of prior art multilateral junctions is the reduction ofthe inner diameter of the wellbore, which vastly complicates thedrilling of subsequent multilaterals, and can require larger verticalwell and mother bore diameters.

Regarding the general well geometries and power/electricity generationaspects of the prior art, Half, in U.S. Pat. No. 6,301,894, issued Oct.16, 2001, teaches a geothermal plant based on a closed-loop subsurfaceheat exchanger. The patent is focused on benefits related to generatorlocation, water conservation and purity and efficiency with multipleloops. The disclosure is silent on techniques to efficiently create theclosed-loop wellbore without using casing.

U.S. Patent Publication, 20110048005, McHargue, published Mar. 3, 2001,teaches a closed loop geothermal system. “The novel approach is tocirculate fluid or gas, here referred to as production fluid, throughsubterranean hot rock formations via a continuous subterranean pipelineformed by cementing continuous pipe along the path made by theintersection of two or more separate bore holes.”

The disclosure is silent on techniques to efficiently create theclosed-loop wellbore without using casing.

Greenfire Energy Inc., in PCT/US/2016/019612, provide, Geothermal HeatRecovery from High-Temperature, Low-Permeability Geologic Formations forPower Generation Using Closed Loop Systems.The text of the case states:

-   “A method or apparatus that uses a fluid in a closed loop well    system to extract heat from geothermal resources that are located in    or near high-temperature, low-permeable geologic formations to    produce power. In some embodiments, the closed loop system may    include one or more heat exchange zones, where at least a portion of    the one or more heat exchange zones may be disposed within a    subterranean region having a temperature of at least 350° C. The    subterranean region may be within a plastic zone or within 1000    meters of the plastic zone, the plastic zone having a temperature    gradient of at least 80° C. per kilometer depth.-   According to some embodiments, methods for producing geothermal    energy described herein may include portions of wells that are not    cased with metal pipe but, instead, the walls of such portions may    be formation rock that has been sealed with hardened sealant and the    well wall in such portions being defined by the boundary of such    hardened sealant which, in some embodiments, will cause the diameter    of the well in such portions to be larger, and in some cases much    larger, than in the metal cased portion of such wells.-   Following emplacement of the closed loop heat exchange system, a    fluid may be circulated through the closed loop geothermal heat    exchange system to heat the fluid and to produce energy with the    heated fluid. For example, the energy extracted from the    subterranean formation may be converted to heat, electricity, or    other usable forms of energy as known to those skilled in the art.-   In addition to determining a temperature profile and the heat    replenishment profile, methods according to embodiments herein may    further estimate a long term viability of a formation for producing    geothermal energy based upon the temperature profile and the heat    replenishment profile. Such an analysis may be performed by    simulating performance of a well as a function of time, taking into    account such variables as temperature, heat flux, plastic    deformation of the formation proximate the well over time, and other    factors, to estimate the changes in energy extraction and energy    conversion efficiencies of the system over time. Such an analysis    may also be performed to compare various portions of a given    formation to determine one or more suitable locations for disposal    of the heat exchange loop.

As described above, embodiments disclosed herein relate to apparatus andmethods for extracting heat from high temperature impermeable geologicalformations, lacking in fractures or porosity either naturally occurringor through stimulation. Contrary to prior teachings and the consensusindicating some degree of permeability, and hence convection, isrequired for effective heat transfer and power production, the presentinventors have found that hot impermeable rock may provide an efficientand sustainable resource for extracting geothermal energy to produceelectricity, for example.

-   A closed loop geothermal heat exchange system may then be disposed    within the subterranean formation based on the determined    temperature profile and the determined heat replenishment profile of    the subterranean formation. Emplacement of the closed loop    geothermal heat exchange system may include drilling, casing,    perforating, cementing, expanding uncased well walls with fractures,    sealing uncased well walls and other steps associated with a    drilling process and emplacement of a well loop therein as known to    one skilled in the art. The emplacing may include, in some    embodiments, disposing a heat exchange zone of the closed loop well    system within a plastic zone or a brittle-ductile transition zone of    the formation. In some embodiments, the emplacing may include or    additionally include disposing a heat exchange zone of the closed    loop well system within a brittle zone of the formation, as well as    stimulating the brittle zone proximate the heat exchange zone.”

It is stated, supra, “Emplacement of the closed loop geothermal heatexchange system may include drilling, casing, perforating, cementing,expanding uncased well walls with fractures, sealing uncased well wallsand other steps associated with a drilling process.”

No teachings regarding the methods, sequence, chemistry or technology isdisclosed regarding sealing lengths of open hole wellbore withoutcasing, maintaining the seal over time, and maintaining wellboreintegrity.

Mortensen, in Hot Dry Rock: A New Geothermal Energy Source,Energy,Volume 3, issue 5, October 1978, Pages 639-644, teaches in an abstractof her article, the following:

-   “A project being conducted by the Los Alamos Scientific Laboratory    is attempting to demonstrate the technical and economic feasibility    of extracting energy from the hot, dry rock geothermal resource. The    system being tested is composed of two deep boreholes drilled into    hot, impermeable rock and connected by a hydraulically produced    fracture. In September 1977, the circulation loop was closed for the    first time and water was circulated through the downhole reservoir    and through a pair of 10-MW (thermal) heat exchangers. A series of    long-term experiments is planned for 1978 in order to evaluate the    thermal, chemical and mechanical properties of the energy extraction    system.”

Building on the exploitation of geothermal energy harvesting, Sonju etal., in U.S. Pat. No. 10,260,778, issued Apr. 16, 2019, claim:

-   “A method for establishing a geothermal energy plant for extracting    thermal energy from a hot dry rock formation with low porosity    wherein a combined supply and return hole (22) is drilled to a first    predetermined depth, then a hole is drilled to a second    predetermined depth forming a lower part (22′) of the combined    supply and return hole, wherein a first manifold zone (8) is defined    at said second predetermined depth, the lower part (22′) of the    combined supply and return hole is extended by drilling with the    same or a smaller diameter hole (1′) to a maximum depth wherein a    second manifold zone (9) is defined, whereby one or more production    hole(s) (P) is/are drilled to form a closed loop between the first    manifold zone (8) and the second manifold zone (9) in which a    working fluid can be circulated, wherein a pipe (5) is positioned in    the combined supply and return hole (22, 22′) and a seal (66) being    installed between said first and second manifold zones (8, 9)    sealing the annulus space (20) between the lower part of the    combined supply and return hole (22′) and the pipe (58) to separate    the supply and return flow.”

In light of the prior art, there remains a need for a method ofextracting heat from a geological formation which can be renderedsuitable in terms of wellbore sealing and maintenance, closedcircuit/loop geometries and multilateral efficiencies for geothermalapplications which is not limited by rock type, permeability inter alia.

The technology of the present invention addresses the imperfections in avariety of technology areas and uniquely consolidates methodologies forestablishing a new direction in the geothermal industry.

SUMMARY OF THE INVENTION

One object of the present invention is to provide significantimprovements to wellbore formation technology generally and in the realmof geothermal energy recovery.

Another object of one embodiment, is to provide a method for drilling awellbore into a formation suitable for geothermal heat recovery,comprising:

-   inducing irreversible formation damage to said wellbore while    drilling said wellbore using at least one of a thermal mechanism,    mechanical mechanism, chemical mechanism and biological mechanism;    and-   forming an interface between said wellbore and said formation    substantially impermeable to fluids.

The use of formation damaging techniques is counter-intuitive in thewellbore formation art and particularly in the geothermal art reliant onfluid migration through porosity, fissures cracks, etc. The presenttechnology has as a first step, mechanisms to seal the fissures, cracksand other areas within the formation facilitating fluid migration.

The methodology employs destructive techniques to reduce permeability ofthe well walls to the point that only conductive heat transfer from thesurrounding rock in the formation transfers the heat into the workingfluid designed to recover the transferred heat.

Immediate benefit evolves from this technique, namely reduced orcomplete lack of use of casings and junctions. This one feature resultsin enormous savings in the drilling process, the latter comprising themajority of the cost associated with geothermal exploitation.

In respect of another object of one embodiment of the present invention,there is provided a method for forming a well with an inlet well and anoutlet well in a formation suitable for geothermal heat recovery,comprising:

-   inducing irreversible formation damage to wellbore extending between    the inlet well and the outlet well while drilling the wellbore using    a chemical mechanism to form an interface between the wellbore and    said formation substantially impermeable to fluids;-   circulating a chemical composition within the wellbore capable of    inducing precipitate formation with the interface to augment the    sealing capacity and mechanical integrity of the interface; and-   circulating a working fluid within the sealed wellbore containing an    interface maintenance additive for maintaining impermeability during    circulation of the working fluid within the well.

Through the innovative selection of chemical compounds and treatmentsequencing, an impermeable interface between the wellbore and thesurrounding formation is synthesized. The result is a lined, selfhealing wellbore which, when integrated into a true surface to surfaceclosed loop circuit, provides an exceptional alternative to frackingbased geothermal operations and those relying on casing throughout thecircuit.

It will be appreciated by those skilled in the art that a plethora ofchemical compositions may be used to effect the synthesis of theinterface. To this end, those compounds in the drilling fluid whichprecipitate with the rock surrounding the wellbore may be used. For thesecond treatment, any suitable compound may be used which reacts withany unprecipitated composition remaining after the first treatment maybe used. Finally, the working fluid for circulation through the loop maybe selected to further react with any fissures, cracks, anomalies, etc.that develop over time in the interface.

The working fluid is selected to optimize the thermodynamic performanceof the geothermal system and to augment the mechanical integrity of thewellbore. Additional treatment operations of the wellbore can beemployed to achieve this subsequent to drilling.

The wells formed using the technology herein can be rendered suitablefor closed-loop geothermal purposes in one operation as opposed to themultiple steps required with existing techniques. Clearly with areduction in the number of unit operations, there is an accompanyingeconomic benefit. This is a major feature of the instant technologywhich elevates it far above the presently employed methods.

From an operations standpoint, irregular or changing conditions duringthe drilling process may be dealt with expeditiously as they evolve.This is another significant feature of the technology, namelyadaptability and flexibility. Since the methodology is predicated upondestructive techniques to form the well in the formation, whichtechniques interfere with the prior art techniques, this technologyinitiates the worst case scenario to render a formation, regardless ofpermeability or geology, suitable as a closed-loop geothermal system.

Regarding an alternative, it is another object of one embodiment of thepresent invention to provide a method of forming a well within ageothermal formation for energy recovery, comprising:

-   drilling an open hole wellbore into a geothermal formation;-   introducing reactive chemical compositions into the wellbore for    reaction to form a fluid impervious interface between the wellbore    and the formation, the interface including unreacted reactive    chemical compositions; and-   introducing a working fluid into the wellbore capable of reacting    with the unreacted reactive chemical compositions for further    formation of the interface.

Since there is effectively a reserve of unreacted reactive composition,the wellbore can self heal in the event of any sealing issue at theinterface. Accordingly, the working fluid not only extracts thermalenergy from the formation for maximum operating efficiency, but furtherensures seal integrity combined with low maintenance.

In furtherance to the clear environmentally responsible methods setforth herein, another object of one embodiment of the present inventionis to provide a method for remediating a wellbore including fracturedsections created by fracturing techniques within an earth formation,comprising:

-   treating wellbore and the fractured sections pore space with a first    chemical composition capable of forming a precipitated impervious    interface at the sections; and-   treating the interface with a second chemical composition for    precipitating any unreacted first chemical cornpositicn to further    seal the interface.

The sealing technologies herein make the remediation possible as well asconversion of existing geothermal operations predicated on fracking.

As such, another object of one embodiment of the present invention is toprovide a method for converting an open geothermal system having atleast one of fractures, unconsolidated rock and sand, an inlet well andan outlet well in fluid communication, to a closed loop geothermal well,comprising:

-   introducing a first chemical composition capable of forming a    precipitated impervious interface between said inlet well and said    outlet well and in said at least one of fractures, unconsolidated    rock and sand whereby a closed sealed loop is formed with said at    least one of fractures, unconsolidated rock and sand, inlet well,    the outlet well, and the area there between; and-   treating said interface with a second chemical composition for    precipitating any unreacted first chemical composition to further    seal said interface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation filtrate volume as a function ofthe square root of time for various fluid formulations;

FIG. 2 is graphical representation of differential pressure andpermeability data as a function of time for the chemical sealing coreflood test delineated in Example 1;

FIG. 3 is a schematic cross section illustration of a well sealedwithout casing in the lateral section between the inlet well and outletwell;

FIG. 4 is a similar view to that of FIG. 3 illustrating a casing stringsegment in the lateral section and the relationship with the sealanttherewith;

FIG. 5 is a similar view to that of FIG. 4 illustrating a sealedwellbore arrangement in a negligible permeability formation withfissures;

FIG. 6 is a schematic illustration of a multilateral arrangement oflateral interconnecting well segments;

FIG. 7 is an enlarged schematic illustration of a sealed multilateralwellbore section referenced in FIG. 6;

FIG. 8 is a schematic illustration of an alternate geothermal wellconfiguration;

FIG. 9 is a schematic illustration another alternate embodiment of ageothermal well configuration;

FIG. 10 is a schematic illustration another alternate embodiment of ageothermal well configuration;

FIG. 11 is a schematic illustration another alternate embodiment of ageothermal well configuration;

FIG. 12 is a schematic illustration another alternate embodiment of ageothermal well configuration;

FIG. 13 is a top view of FIG. 12;

FIG. 14 is a schematic illustration another alternate embodiment of ageothermal well configuration;

FIG. 15 is a schematic illustration another alternate embodiment of ageothermal well configuration;

FIG. 16 is a cross section of a drilled wellbore within a highpermeability formation illustrating the reserve of unreacted sealant;

FIG. 17 is a view similar to FIG. 16 illustrating the transformation ofthe wellbore interface subsequent to circulatory contact with theworking fluid;

FIG. 18 is a schematic cross section illustration of a drilled wellborein a low permeability formation and the interface with the surroundingformation;

FIG. 19 is a schematic illustration of a power cycle implementation ofthe geothermal wellbore methodology;

FIG. 20 is a schematic illustration of an alternate embodiment of FIG.19;

FIG. 22 is a schematic illustration of an integrated geothermal circuitincorporating a turbine and generator directly driven by the geothermalworking fluid;

FIG. 23 is a schematic illustration of an alternate embodiment of FIG.22.

FIG. 24 is a graphical representation of temperature data over distancefor different working fluids;

FIG. 25 is a schematic illustration of a W shaped or daisy chaingeothermal well configuration;

FIG. 25A is an enlarged view of the interconnecting well formation ofFIG. 25.

FIG. 26 is a schematic illustration of alternate embodiment of FIG. 25;

FIG. 27 is a schematic illustration of alternate embodiment of FIG. 25;and

FIG. 28 is a schematic illustration an alternate embodiment of FIG. 25.

Similar numerals used in the Figures denote similar elements.

The technology has applicability in the geothermal technology andremediation of geothermal sites.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In overview, the technology herein relates to wellbore formation anddesign with examples for closed-loop geothermal wellbores. The designaspect includes:

-   i). sealing the wellbore while drilling;-   ii). augment the seal with a chemical treatment subsequent to    drilling; and-   iii). displacing the drilling fluid, post drilling with a    circulating working fluid which augments and maintains the seal with    self-healing any remaining or generated permeability and maintains    wellbore integrity.

The flexibility of the approach allows each of these aspects to be usedseparately, depending upon the specific geology of the formation,however, they are most effective when integrated and working in concertto create and maintain a closed-loop geothermal system.

The wellbores can be any number of configurations, such as a singleU-tube with an inlet/outlet, a U-tube wherein the inlet and outlet wellare located on the same surface lease, a “tube-in-tube” configurationwhich could be vertical, deviated, or horizontal, and include“daisy-chaining” several of these wellbores together, L shaped, etc.These are examples and are not intended to be limiting. Other suitablearrangements will be appreciated by those skilled in the art.

The aspects noted above are particularly effective when used to formmultilateral wellbores wherein a plurality of laterals are connected toa vertical well, typically in a U-tube configuration with multiplehorizontal laterals connecting a vertical cased inlet well and avertical cased outlet well. When used in a multilateral configurationseveral advantages are realized not recognized in the art. Theseinclude:

-   i) The laterals can be initiated, drilled, and completed open hole    avoiding the expense and time associated with installing casing:

ii) The “open hole” junctions can be created and sealed while drillingin a single step. This avoids complicated mechanical junctions, cementplacement, drilling out plugs or metal sections, multiple trips tosurface, and in general the complications and expense associated withintricate downhole processes and resulting delay in forward drilling;

iii) There is no material reduction in inner diameter which enablesunlimited number of laterals to be drilled;

iv) There is no reduction in thermal conductivity created by aninsulating cement layer or stagnant annulus between steel liner androck; and

v) Enablement to re-enter multilaterals with magnetic ranging equipmentto intersect other lateral wellbores and create a closed U-tube wellboreconfiguration.

In respect of the sealing while drilling aspect, this may beaccomplished by including additives within the drilling fluid itselfthat cause irreversible formation damage and reduce the permeability tozero or negligible levels.

The additives may be biological growth accelerants such as thetechniques used in Microbial Enhanced Oil Recovery, physicalparticulates that create an impermeable filter cake, or chemicalsealants that react upon contacting and penetrating into the geologicalformation such as time-set or thermally-set resins and epoxies, gels,and polymers.

Another method for sealing wellbores while drilling is to thermally sealthe face of the rock with extremely high temperatures that melt thewellbore wall, for example by using a high temperature plasma orlaser-based drilling bit.

The preferred method is to use a chemical sealant, for example analkali-silicate based drilling fluid with a pH greater than 10.5, thatremains liquid within the wellbore, but precipitates into a solid uponcontacting and penetrating into the rock. The technical function of thedrilling fluid is different in permeable rocks (for example sandstone orfractured basement) relative to impermeable rocks such as hard shales orsiltstones. In permeable formations the liquid alkali-silicate drillingfluid penetrates any available flow paths prior to reacting and settinginto a solid. The resulting solid precipitate is impregnated and fusedinto the pore space and natural fractures within the rock itself andcreates a fluid impervious barrier between the wellbore and thegeological formation.

In contrast, in rocks with near zero permeability such as shale, thefunction of the drilling fluid is not to seal off permeability—the rockalready has none. Instead, the function of the drilling fluid is toprovide a mechanical and chemical barrier between the rock and wellboreand to fill in any natural fractures, fissures, or cleave planes. Theend result is the same, to create a fluid impervious barrier between thewellbore and the geological formation.

The sealant may also be used to consolidate unconsolidated sands,increase the compressive strength of the rock, and prevent sandproduction and sloughing.

As is known, soluble silicates contain three components, namely silica,alkali, and water. Silica (silicon dioxide, SiO₂), is the principalconstituent of soluble silicates and is stabilized by an alkali. Thealkali may be selected from sodium, potassium, or lithium oxide (Na₂O,K₂O, or Li₂O) and is responsible for maintaining the solubility of thesilica.

Suitable silicates include potassium, sodium and sodium aluminosilicate.These products are available in both liquid and powdered forms.Silicates are desirable for use in this technology since they canundergo distinct types of chemical reactions, namely gelation (drop inpH), which is the self-polymerization or condensation of solublesilicate structures to form a hydrous, amorphous gel structure ofsilicate. Gelation is brought on by a drop in pH with polymerizationbeginning to rapidly occur at pH below 10.5.

Another type of reaction the silicates can undergo is precipitation withcations such as calcium. Precipitation of silicate is the cross-linkingof silicate molecules by multivalent cations (i.e. Ca⁺², Mg⁺², Al⁺³,Fe⁺³, etc). These cations are present in the formation water—a drillingfluid to formation fluid interaction therefore results in solidprecipitation within the pore space.

A further type of reaction the silicates undergo is dehydration. Aswater is removed from liquid silicate, the silicate progressivelybecomes tackier and more viscous and eventually becomes a glassy film.These are the reactions that occur in the near wellbore as filtrate fromthe drilling fluid mixes with fluids within the rock matrix.

Silicates are especially attractive to this geothermal application sincethey are a stable sealant at ambient conditions and at extremely hightemperatures. For example, alkali-silicate and sand is used attemperatures of 650° C. and above in the foundry and liquid metalcasting industry, and this basic chemical reaction is also employed toseal concrete structures at ambient temperature.

The alkali-silicate drilling fluid is formulated to be solids free andlow viscosity to maximize wellbore fluid invasion and spurt loss tochemically seal the wellbore. For multilateral horizontal well segmentsfriction is a significant challenge, so a lubricant is added that iscompatible with silicate brine and does not materially interfere withthe sealant properties.

The concentration of active alkali-silicate can be from 0.3%-10% butmore likely from 3%-6% by mass in water. The optimum concentrationdepends somewhat on the geological properties such as in-situ brinecomposition and temperature. Higher rock temperatures can cause a delayin the precipitation reaction. Likewise, formations where the in-situbrine has a low concentration of multivalent cations, for example, below1000 mg/L, cause a slower reaction. Therefore as rock temperatureincreases and multivalent cation concentration decreases, theconcentration of alkali-silicate should be increased.

Ancillary benefits of a silicate brine include an enhanced rate ofpenetration, (ROP), and increased bit life.

The physical properties of the combined rock/sealant material arelargely derived from the rock but can be modified by carefully selectingthe properties of the sealant. A thermally conductive additive may beincluded with the drilling fluid, such as graphene nano particles, sothat the resulting sealant has a high thermal conductivity.

The energy output of a closed-loop geothermal system can be determinedusing a thermodynamic wellbore model consisting of a discretizedwellbore with multiple thermal resistances between the fluid temperatureand the far-field rock temperature. Each discretized segment has anenergy and mass balance performed, where fluid properties andcalculations are handled with an equation of state thermodynamicspackage. The heat transfer resistances include the rock, cement, steelcasing, and convective heat transfer resistance within the wellboreitself.

As a quantitative example, using a 7″ cased and cemented well in contactwith a geological formation with a thermal conductivity of 3 W/ m K, thethermal resistances after 5 years of operation for the rock, cement,casing, and pipe flow convection are, respectively, 2.2E-02, 2.1E-03,2.9E-05, and 5.0E-5. The heat transfer is dominated by radial conductionthrough the rock, and all other thermal resistances are negligible incomparison. Using the chemical sealant described herein, there are noresistances to heat transfer from casing or cement, so the thermalefficiency is approximately 9% higher than prior art methodology. Byenhancing the thermal conductivity of the bulk rock/sealant material,heat transfer can be increased further.

The alkali-silicate sealant can be further enhanced by incorporating asolid particulate that is formulated to become chemicallyembedded/bonded within the alkali-silicate precipitate, to improve sealperformance and mechanical integrity. Reinforcing materials such asexfoliated fly ash, surface-activated graphene and graphene oxide,carbon fibres, and others may be incorporated into the drilling fluid.These may be in a nano-dispersed or micro-dispersed state and chemicallybond with the precipitated silica.

After the initial seal is made while drilling, the integrity of the sealis tested. Typically, this is done by pressurizing the wellbore systemand monitoring the rate of depressurization, if any, as is common in theindustry. Another method is through long-term measurement of theleak-off rate during circulating operations. In this case, the drillingfluid is removed and replaced with the working fluid whose primarypurpose is to transfer energy to surface, and the leak-off rate ismeasured during regular operations.

While the seal will be substantially complete after drilling, there maybe some small areas with minor permeability remaining, such as fracturedzones or highly permeable channels that were not sufficiently sealedwhile drilling. Therefore, the seal can be augmented using a chemicalflush or treatment prior to commencing or returning to normaloperations.

When employing alkali-silicate drilling fluid as described previously,the drilling fluid reacts with the in-situ formation fluid to gel andeventually solidify into a hard, high strength solid. These reactionshappen at the mixing interface between the silicate drilling fluid andthe formation fluid. In a high permeability channel or fracture, thedrilling fluid may be migrating through the formation so quickly thatthe formation fluid is displaced away from the wellbore and the mixinginterface is pushed substantially into the rock or the formation brinemay be extremely fresh causing the silicate to gel but not completelyprecipitate.

In these scenarios, a partial or substantial seal is achieved deepwithin the rock, but the near-wellbore region contains “unspent” orunreacted liquid alkali-silicate drilling fluid and no further formationbrine with which to react. Therefore, the purpose of the chemical flushis pump a chemical treatment through the wellbore system with sufficientpressure to cause leak-off from the well bore into the near-wellboreformation, contact the unspent liquid alkali-silicate remaining from thedrilling process, and initiate the precipitation reaction. Suitablechemicals are calcium chloride brine, acids, CO₂, surfactants, esters,among others known in the industry.

In another embodiment to augmenting the seal, a chemical treatment maybe pumped through the wellbore system with sufficient pressure to causeleak-off from the wellbore into the near-wellbore formation, where thechemical treatment consists of “plugs” or volumes of alkali-silicatefollowed by a reacting chemical consisting of calcium chloride brine,acids, CO₂, surfactants, esters, or others known in the industry. Thetwo chemicals can be alternatively pumped several times resulting insubstantial mixing in the near-wellbore region. The volumes ofalkali-silicate and reactant may be separated with a spacer to preventmixing within the wellbore or be in direct contact.

Turning to maintaining the seal arid wellbore integrity duringoperation, the drilling process, as is commonly employed in the oil,gas, and geothermal industry, requires maintenance of wellbore integrityand a partial wellbore seal (i.e. a filtercake), for a temporaryduration until casing is cemented in the hole or a liner is installed.The open hole (prior to installing casing or liner) wellbore integrityand partial seal is created by proper engineering and application of thedrilling fluid.

In contrast, the invention disclosed herein requires maintaining an openhole seal and wellbore integrity for the operational life of thegeothermal asset which is typically 50 years or more.

In addition to creating the seal while drilling and optionallyaugmenting the seal with a separate chemical treatment, the operationalworking fluid itself has a key role in maintaining the seal andmaintaining wellbore integrity. The primary function of the workingfluid is to transport energy from the subsurface rock to surface whereit is directly used or converted into electricity or cooling. Therefore,the working fluid must have key physical properties for energy transferand to maximize thermodynamic efficiency of the system. For example, thefluid may have at least one property selected from the group comprising:

-   a) a substantially nonlinear temperature enthalpy relationship    within the lateral interconnection section between the inlet well    and the outlet well at pressures greater than 10MPa and temperatures    less than 180° C. to maximize the temperature differential and heat    transfer between the fluid and the surrounding downhole heat source;-   b) capable of undergoing a pressure-sensitive reversible reaction    which is endothermic at elevated pressure and exothermic at pressure    lower than the elevated pressure;-   c) a fluid mixture containing a chemical absorption reaction which    is endothermic within the lateral interconnection;-   d) an aqueous electrolyte solution with temperature and pressure    dependent solubility, resulting in an endothermic effect within the    lateral interconnection;-   e) water-based fluid containing a turbulent drag reducing    composition;-   f) supercritical fluid such as CO₂;-   g) ammonia-ethane mixture; and-   h) functional combinations of a) through g)

In addition to maximizing thermodynamic efficiency, the working fluidalso has many properties of a drilling fluid, namely to:

-   i) transport solid particulates that may collect in the wellbore to    surface where they are removed, typically with a settling tank,    filter, or hydrocyclone;-   ii) maintain a seal of the wellbore so that it is substantially    impermeable to fluids; and-   iii) maintain wellbore stability and integrity.

In one embodiment, the seal may be maintained by providing solidparticulates within the working fluid that form a filter cake along theborehole wall or bridge and plug natural fractures. These particulatesmay be carbon fibres, mineral fibres, cellulose fibres, silica, fly ash,graphite, graphene, graphene oxide, calcium carbonate, bentonite, orother particulates known in the industry. These solids are typicallyadded at between 0.5 and 2.0 weight % of the working fluid if its waterbased, and equivalent volume concentration for other working fluids.

When employing alkali-silicate drilling fluid as described previously,the drilling fluid reacts with the in-situ formation fluid to gel andeventually solidify into a hard, high strength solid. These reactionshappen at the mixing interface between the silicate drilling fluid andthe formation fluid. In a high permeability channel or fracture, thedrilling fluid may be migrating through the formation so quickly thatthe formation fluid is displaced away from the wellbore and the mixinginterface is pushed substantially into the rock or the formation brinemay be extremely fresh causing the silicate to gel but not completelyprecipitate. In these scenarios, a partial or substantial seal isachieved deep within the rock, but the near-wellbore region contains“unspent” or unreacted liquid alkali-silicate drilling fluid and nofurther formation brine with which to react. Therefore, another methodto maintain a seal is to include a reactant additive that uponleaking-off from the wellbore into the near-wellbore formation, contactsthe unspent liquid alkali-silicate remaining from the drilling processand initiates the precipitation reaction.

By definition, any areas of the wellbore where permeability remainsafter drilling will have had considerable influx of alkali-silicate andcontain unspent liquid alkali-silicate in the near-wellbore formation.Therefore, including a reactant within the working fluid will naturallyseal off the remaining permeable sections. Suitable chemicals arecalcium chloride brine, acids, CO₂, surfactants, esters, and othersknown in the industry.

To maintain wellbore stability and integrity, in addition to sealing therock, the working fluid must exert enough pressure on the formation toprovide sufficient compressive strength to prevent breakouts, sloughing,and partial collapse of rock into the wellbore. The pressure that anoperational working fluid provides can be calculated using an integratedthermodynamic wellbore model that includes an equation of state toaccount for phase changes, fluid property changes with pressure andtemperature, and hydraulic frictional losses. When designedappropriately, the working fluid must supply the minimum compressivestrength across the entire wellbore, either by applying a sufficientlyhigh pressure at the top of the inlet well (pressurized fluid), or bymodifying the density of the working fluid. Fluid density can beincreased through addition of weighting agents such as barite or throughsoluble salts, among other techniques known in the industry.

Another method to maintain wellbore stability is to include a shaleinhibitor chemical within the working fluid. This chemical has thefunction of arresting the hydration, swelling and disintegration ofclays and shales, and is a common additive in drilling fluids. Suitableadditives are amine-based chemicals, latexes, or an aqueous solution ofpotassium salts, among others known in the industry.

The combination of the above additives and functions results in aworking fluid that not only transports energy to surface efficiently,but also reinforces and maintains the wellbore seal, “self-heals” anygenerated permeability, and maintains wellbore stability and integrity,to preserve a closed-loop geothermal wellbore system that issubstantially impermeable to fluids.

Of critical importance is the requirement that the sealant additives donot interfere with the thermodynamic properties of the working fluid. Inone embodiment, the working fluid consists of water, a commerciallyavailable corrosion inhibitor at between 1 and 10 L/m3,potassium bromideat between 0.05 and 0.3 mol/L, cetyltrimethylammonium surfactant atbetween 3 and 7 mM, sodium salicylate at between 8 and 16 mM, andcalcium carbonate solid particulates at 0.5 weight %.

The solution described above maintains greater than 60% turbulent dragreduction over a temperature range suitable for direct-use geothermalheat supply, which is critical for thermodynamically efficientoperation. It also has over 40% recovery when tested according to API RP13i Procedures for Shale Dispersion by Hot Rolling, reacts with unspentalkali-silicate to form a strong solid material, and the calciumcarbonate particles bridge and plug natural fractures and matrixpermeability.

In another embodiment, the working fluid itself is simply a modifiedalkali-silicate brine.

In another embodiment, the working fluid is supercritical CO₂ which isof particular value since in many geothermal scenarios supercritical CO₂has thermodynamic efficiency superior to water, and it is also anexcellent reactant to cause alkali-silicate liquid to solidify into astrong solid material.

The various sealing mechanisms will now be delineated in the followingexamples.

EXAMPLE 1 Chemical Sealing

Initial testing of the sealing capabilities of the silicate system wasperformed in a permeability plugging apparatus.

Permeability Plugging Apparatus Tests:

-   20 μm, 3000 mD discs (provided by OFITE) were soaked in a 30%    calcium chloride solution overnight (approximately 16 hours) in    order to fully saturate the pores with the brine and create a    ‘severe case’ in situ fluid for the silicate drilling fluid with    which to react.-   Permeability plugging tests (PPT) were run in accordance with OFITE    Instruction manual and API RP 13i—Recommended Practice for    Laboratory Testing of Drilling Fluids—250 mL of the test fluids    described below was transferred to the PPT cell and a pre-soaked    disc was placed in the apparatus. The drilling fluid was allowed to    contact the disc for 45 minutes prior to pressurizing the apparatus    and beginning the test-   The tests were performed for 30 minutes at room temperature and 500    psi-   Filtrate volume was recorded after 1, 5, 7.5, 15, and 30 minutes

FIG. 2 is a plot of some data that is typical of the test on a ¼″ thickfiltration disc. A polymer control fluid was flowed through and there isno material reduction of the filtrate volume. When different types ofsilicates were added, the filtration rates were slowed drastically asprecipitation occurred. Note that the permeability has been nearlyeliminated even in a ¼″ thick disk with 3000 mD of permeability.

Fluid Preparation:

-   1000 mL of 5 kg/m3 polymer fluid was prepared by mixing xanthan gum    (Kelzan XCD™) into fresh water for approximately 30 minutes using a    Silverson Mixer at moderate shear rate.-   The control fluid was the polymer fluid above.-   Formulation A, 30 mL of Ecodrill™ 317, a commercially available    product from PQ Corporation, was combined with 270 mL of the polymer    fluid above to produce a 300 mL portion of 3% active soluble    potassium silicate.-   Formulation B, 30 mL of Ecodrill™ K45, a commercially available    product from PQ Corporation, was combined with 270 mL of the polymer    fluid above to produce 300 mL portion of 3% (VN) active soluble    silicate.

The total PPT Volume was 273.8 mL for the Formulation A, a spurt loss of257 mL was calculated, and a Static Filtration Rate of 3.1 mL/min wascalculated. The total PPT Volume was 103.8 mL for the Formulation B, aspurt loss of 103.8 mL was calculated, and a Static Filtration Rate of3.7 mL/min was calculated. Values calculated using formulas expressed inAPI 13i.

Core flood/regain permeability/core damage studies were also conducted.These types of tests are often used to study the effects of a drillingfluid or drilling fluid additive on the permeability of a core obtainedfrom a target production zone of interest. Usually the object of thestudy is to minimize the damage or maximize the regain permeability. Aninitial permeability is established and measured by saturating the corewith native brine, oil, or some brine/oil mixture, and flowing theformation fluid(s) through the core at pressure at reservoir pressureand temperature conditions. A test fluid is then injected across theface of the core for a certain period of time the volume of filtrate,invasion of fluid, and thickness of filter cake may be measured.Formation fluids are then injected in the reverse direction of flow todetermine the extent to which the permeability may have decreased oreven increased after exposure to the test fluid. In this study, the aimwas to damage the cores by means of gelation and precipitation reactionsof the silicate test fluids with the synthetic brine-saturated cores.

Core flood/regain permeability/core damage studies were carried out asfollows:

Berea Sandstone cores with permeability approximately 30 mD weresaturated with synthetic brine under vacuum and tested with a 3%solution of potassium silicate and containing 2% of a specialtylubricant.

-   Test procedures, parameters and results are set forth below.

Procedure:

-   1) Plugs were weighed and pre-saturated with brine for 1 week under    15 inHg vacuum.-   2) Placed in core flow and permeability to brine was measured.-   3) Potassium silicate mud was mixed and heated to 95° C.-   4) Mud is injected into core at continuous rate of 3 mL/min.-   5) Pressure is monitored over time.-   6) Differential pressure builds exponentially over time until    ˜2500psi. Breakthrough of fluid is observed.-   7) The core does not completely plug off, however ˜99% of    permeability is lost.-   8) Effluent is collected to determine fluid displacement (depth of    invasion).

Parameters:

-   Instrument: Chandler Formation Response Tester-   Core Plug: 1.5″×3.0″ Sandstone-   Temperature: 95° C.-   Test Fluid: Potassium silicate at 3% with 2% lubricant-   Pore Volume: 16.78-   Initial Permeability: 28.32 mD to brine-   Permeability after mud treatment: 0.197 mD-   Permeability Reduction: >99%-   Flow rate: 3 mL/min-   Brine composition:-   NaCl-230.303 g-   CaCl₂-79.054 g-   KCl-8.346 g-   MgCl₂-13.79 g

Shale dispersion testing was then executed to determine the ability ofalkali-silicate solutions with lubricant to seal and provide mechanicalintegrity to shale samples. The methodology is according to API RP 13iProcedures for Shale Dispersion by Hot Rolling as follows:

-   an approximately 2 kg piece of Pierre Shale was crushed to yield    approximately 900 g of −5/+10 Mesh (2-4 mm) pieces. Pierre Shale is    much more reactive and susceptible to water than the mature, hard    shale formations typically present at the depths suitable for    geothermal. It was chosen as a conservative baseline, actual    performance with mature shales will be better.    -   The −5/+10 mesh pieces were sieved using ASTM sieves and a        Ro-Tap sieve shaker for 2 minutes    -   Approximately 10 g of shale was placed in 250 mL of test fluid    -   The samples were rolled for 24 hours at 120° C.    -   The samples were then poured into a 20 mesh screen after rolling    -   Aging cells were rinsed with inhibited fluid (7% KCl) to        dislodge any material adhering to the inside walls    -   The total amount of material recovered on a 20 mesh was dried to        a constant mass at 100 C in an oven    -   Each sample was then re-sieved and the mass of the −5/+10        fraction was recorded

Results for several different fluid formulations are presented below.

Initial Mass Total Recovered (g, −5/+10 Recovered Mass (g, % No Samplemesh) Mass (g) −5/+10 mesh) Recovery 1 Water 10.025 2.027 0.113 1.1 2 3%(v/v) Potassium 10.041 9.895 9.799 97.6 Silicate 3 3% (v/v) Potassium10.007 10.164 9.657 96.5 Silicate + 2% Lubricant 4 Mineral Oil 10.0119.251 8.501 84.9 5 7% KCl 10.054 9.015 7.307 72.7 6 10 L/m³ Amine 10.0026.961 5.759 57.6 7 Working Fluid 10.175 7.102 4.514 44.4 Composition

Recovery of over 97% is achieved, indicating excellent sealing andstrengthening of the shale. Mineral oil has no reactivity with shale,yet only recovered ˜85% of the mass. The loss of mass is due tomechanical degradation during rolling. Therefore, the high 97% recoveryindicates that not only is a chemical seal form, but a mechanicalhardness improvement is also realized. The working fluid with shaleinhibitor added also has a 44% recovery which is substantially improvedfrom fresh water which has only a 1% recovery.

EXAMPLE 2

A working fluid was tested consisting of water, a commercially availablecorrosion inhibitor, potassium bromide, cetyltrimethylammoniumsurfactant, sodium salicylate, and calcium carbonate solid particulatesat 0.5 weight %.

Measurement of pressure drop (i.e., drag) and characterization of theturbulent flow was tested using a 2″200 L capacity heated flow. The loopis equipped with a centrifugal (GIW, LCC-M 50-230) and a progressivecavity (Moyno™, 2F090) pump with high and low shear, respectively. Themaximum Re number reaches 500,000 and the loop can operate with 15%volumetric concentration of solid. Pressure drop was calibrated withfresh water and compared to frictional pressure drop at the same flowrate using the working fluid. A turbulent drag reduction of 63% wasachieved over a temperature range suitable for direct use heatapplications.

To test reactivity with unspent alkali-silicate in the near-wellbore,Ecodrill™ 317, a 29.1% active solution of 2.5 ratio SiO₂:K₂O was mixedinto samples of the working fluid. NaOH was used to adjust to a pH of11-12, and the alkali-silicate solution was injected into samples of theworking fluid under gentle agitation to produce a 3% (v/v) and 1% (v/v)solution. These low concentrations were chosen to conservativelyrepresent the near-wellbore unspent alkali-silicate drilling fluid. Ineach case the addition of the silicate solution into the working fluidcaused precipitation, and after 24 hours the silicate was solidified.The results demonstrate the working fluid will reinforce and augment thewellbore seal so that it is substantially impermeable to fluids.

To assess the ability of the working fluid to maintain wellboreintegrity and stability, a modified shale dispersion test was performed.The test methodology involves 2 shale dispersion runs back-to-back withthe same sample. First, the sample is hot rolled in the sealant, asdescribed above, then re-soaked in the working fluid to determine shalemechanical strength and chemical isolation after sealing. After theinitial shale dispersion run with the drilling fluid sealant, thesamples are dried, weighed, and immersed in the working fluid chemistryand rolled for 24 hours.

The samples were then poured into a 20 mesh screen after rolling, andthe total amount of material recovered on a 20 mesh was dried to aconstant mass at 100 C in an oven. Each sample was then re-sieved andthe mass of the −5/+10 fraction was recorded and compared to the mass ofthe sample after sealed and dried. Interestingly, the results frommultiple runs showed over 96% recovery of mass, indicating excellentability of the working fluid to maintain wellbore integrity.

EXAMPLE 3 Mechanical Method

In one embodiment, the mechanism may be effected by adding solidparticles to the drilling fluid which migrate naturally into the porespace/fractures to reduce permeability. This is generally known as losscirculation material (LCM)

The solid particles may be granular materials, fibrous materials andflaked materials and combinations of these and be present (dispersedthrough drilling fluid) in sizes necessary to reduce permeabiliyl.Suitable sizes may be nanometer to millimeter size.

Abrams' rule and/or Ideal Packing Theory concepts are useful toestablish the most suitable materials. Abrams' rule proposes particlesize of the bridging agent should be equal to or slightly greater than ⅓the medium pore throat size of t targeted form ion.

The ideal packing theory proposes a full range of particle sizedistribution to effectively seal all voids, including those created bybridging agents.

Particles may also be sized to penetrate into the pore space beforebridging.

Additionally,drill cuttings can augment the LCM and serve as pluggingmaterial

Any of these LCM products could he utilized remediating wellbore leaksafter the ng process is completed. Further viscous sweeps with LCM maybe pumped at reduced rate through the open hole section to allowinvasion of the LCM and seal any leaks,

Finally, solid silicates (possibly encapsulated) may also provide aneffective chemical/mechanical combination mechanism for sealing thereservoir.

EXAMPLE 4 Biological Method

Microbial Enhanced Oil Recovery (MEOR) is an engineering field whichmanages the design, growth, and stimulation of microorganisms toincrease oil recovery. Most deep geological formations contain anaerobicbacteria within the pore space. These bacteria have a very low supply ofenergy and nutrients compared to near-surface microbes, and thus havelow population densities.

One MEOR technique is to treat the indigenous microbes with nutrients tospur their growth and eventual plugging of the rock porosity withbiological material. The nutrients may be any chemistry but typicallyinclude potassium nitrate and monosodium phosphate. As bacteria growthis exponential, if supplied with sufficient raw materials and suitableconditions, bacteria can be induced to grow and completely plug off thepore space in which they reside, causing the rock to be substantiallyimpermeable to fluids.

Another technique is to introduce new microbes to the rock formation andsimultaneously supplying them with nutrients. These microbes may beengineered to grow only at a certain temperature and so can be activatedby injecting into a hot formation.

Either technique can be applied to a conventional drilling fluid,causing the rock to be substantially impermeable to fluids, and form aclosed-loop geothermal system.

EXAMPLE 5 Thermal Method

Geological formations have varying chemistry and thus, varying meltingpoints, although most sedimentary formations melt at 1200° C. or below.Several technologies are in the research, development, and testing phasewhich can penetrate through rock using thermal disintegration ratherthan mechanical contact.

One method is to create a plasma either through electric current ornuclear power. The plasma melts the rock and enable continuous drilling.

Another method is to fire lasers onto the surface of the rock,increasing the temperature until the rock begins to spall, disintegrate,and eventually melt.

Another method is to fire high velocity projectiles which release enoughenergy on impact to increase temperature by hundreds of degrees.

Each of these techniques have the ability to melt porous and permeablerock while drilling, which can then be cooled and annealed to form ahard, durable barrier substantially impermeable to fluids.

Having discussed the method details of the technology, reference willnow be made to specific implementations with reference to the figures.

Referring now to FIG. 1, shown is a graphical representation of filtratevolume as a function of the square root of time for differentformulations.

FIG. 2 is a graphical representation of differential pressure andpermeability data as a function of time for the chemical sealing coreflood test delineated in Example 1.

FIG. 3 is cross section of a well having an inlet well 10 having surfacecasing 12 for groundwater protection. Intermediate casing 14 is cementedinto position as illustrated. All of these components are known in theart. Extending from intermediate casing 14 is the lateral section 16which does not include casing in this example, but rather is the opensealed wellbore. Pore space surrounding the lateral section 16 is sealedwith sealant as described herein previously. The sealed pore space isreferenced by numeral 18, the sealed lateral section is continuous tointermediate casing 14. The latter casing then continuously connectswith outlet well 20. The outlet well is completed with casing 12.

FIG. 4 illustrates an alternate scenario. In this example, the lateralsection 16 may be intermittently sealed resulting in unsealed rock face22. In this situation, a casing 24 is shown as a liner, meaning lackingcementing. The liner 22 thus ameliorates the unsealed rock face andmaintains a continuous circuit from the inlet 10 to the outlet 20. Thismay be used in conjunction with continuously sealed sections. This willdepend upon the specific geology of the formation.

In respect of the sealed areas in porous or fractured rock, the sealantis not fused with the rock face, but rather is embedded within the rockin the chemical example discussed supra. Generally, FIGS. 2 and 3 depicthard rock.

Referring now to FIG. 5, shown is an example where the well is disposedwithin a less permeable section within a formation, an example of whichis a sedimentary shale or mudstone section. In this scenario, theformation may have infrequent fissures, fractures, cleave planes etc.generally denoted by numeral 26. A chemical liner 28 may be employed tocomplete the continuity between the inlet 10 and the outlet 20 with thechemical liner composition 28 filling the fissures, fractures and cleaveplanes as shown.

Turning to FIG. 6, a first example of a well configuration is shown. Inthe example, each of the inlet 10 and outlet 20 include conventionalcased sections 30 which communicate with the inlet 32 and outlet 34 of amultiple lateral well system 36. The system is disposed within ageothermal formation 38. The system 36 includes a plurality of lateralwells 16, which may be partially cased depending upon the situation asoutlined with respect to FIGS. 3 and 4. Any number of well systems 36may be employed in the formation 38. This is represented by numeral 6 nvertically and horizontally, with the “n” being indicative of any numberof further wells in the shape of the system 36 or any other suitableconfiguration.

The inlet 32 and outlet 34 are integrated with the cased sections 30 ina multilateral junction which will now be referenced in the advancingFigures.

FIG. 7 illustrates one possible multilateral arrangement. Inlet 32connects with a sealed multilateral wellbore junction 40 from which thelateral sections 16 continuously extend. The lateral sections 16 arespaced apart from one another to maximize thermal recovery from withinthe formation 38 (FIG. 6). The lateral sections 16 may include casing asdiscussed with respect to FIGS. 3 through 5. The outlet 34 of the system36 will include a similar junction 40 (not shown).

Turning to FIG. 8, an L- shaped well configuration is shown, generallydenoted by numeral 42. In this example, the well has an extendingsection 44 having a terminal end 46 open hole wellbore sealed as withprevious examples. An insulated tube 48, extends within the well forfluid delivery. The extending section 44 may be at any selected angle.

FIG. 9 illustrates a vertical orientation example.

In FIG. 10, a W-shaped well is provided represented by numeral 50. Thesurface is denoted by numeral 52. In this example, output from one wellbecomes the input of the other well. Flow direction is shown with thearrows. The pattern can be repeated for additional cycles. In thisexample, the open hole wellbores 16 are sealed as discussed with theprevious figures and may incorporate alternating patterns of casedsections with simply sealed wellbore sections. This will depend on theformation geology.

FIG. 11 illustrates a further variation of a multilateral system similarto that initially referenced in FIG. 3, which combines the inlet andoutlet conduits into a single wellbore, the disposition of themultilateral section may be at any angle within a formation. In thisexample, the lateral sections 16 converge at terminal end 54.

FIG. 12 is a side view of a single site arrangement 56 where the inletwell 10 and outlet well 20 are generally proximate. The fluid circuit isshown for the lateral sections 16. As with the previous examples, theopen hole well bores are sealed while drilling is conducted with thesurrounding pore space sealed during the procedure. The numeral 12 ncarries the same meaning as that ascribed to 6 n referenced in FIG. 6.

A further variation is illustrated in FIG. 13. A top view is shown of amultiple lateral well arrangement. The individual wells 16 of theplurality shown, share a common inlet well 10, extend out within thegeothermal zone of the formation (not shown in this Figure) and returnin a closed circuit to a common outlet well 20. Flow direction is shownwith arrows, and flow can be isolated to each individual loop or daisychain among the separate loops. This is advantageous for a smallfootprint while allowing for maximum thermal mining within a geothermalzone.

FIG. 14 depicts a further variation where there are plural wellarrangements provided while maintaining the small footprint attributedto the embodiment of FIG. 13.

FIG. 15 incorporates the multilateral well system 50 in combination withthe well configuration initially presented in FIG. 13. In thisconfiguration, two discrete single sites 56 can span a large area withminimal surface invasiveness to mine a large underground geothermalarea. Flow direction is shown with arrows, and flow can be isolated toeach individual loop or daisy chain among the separate loops. Theeffectiveness of the sealing technology discussed herein permits theflexibility to provide for such hybrid configurations. This, in turn,allows for thermal mining in a wide variety of geologic situationsallowing for yet another degree of freedom in practicing the methodsherein.

In more focused detail, FIG. 16 illustrates a cross section of a drilledwellbore, also referenced herein as a lateral section 16. The geothermalformation 38 in this example is a high permeability formation. Due tothe permeability, the sealant spreads out within the pore space in theformation and immediately proximate the wellbore 16 remains unreacted,referenced by numeral 60. Outwardly from the unreacted sealant area ispore space sealed with sealant which, as in the previous examples, isdenoted by numeral 18.

FIG. 17 illustrates the result of exposing the wellbore from FIG. 16 toworking fluid. Subsequent to this treatment, the formation areasurrounding the wellbore becomes sealed forming an impervious interfacebetween the interior volume of the wellbore itself and the formationsurrounding it. This is particularly advantageous since the sealedwellbore 16 is surrounded by a reserve of unreacted sealant 60. In theevent of wellbore seal compromise from seismic activity or otherdeleterious activity, the wellbore can sustain its integrity and sealingcapacity by self healing with the reaction between the reserve ofavailable reactant and the working fluid. This clearly has verysignificant advantages in terms of reducing operating and maintenancecosts over time which, of course, greatly mitigates the initial capitalexpenditure associated with initially synthesizing the well system.

In respect of rock with low or average permeability, an example of whichis granite, mudstone, or shale, the pore space, fractures, fissures,cleave planes etc. may be filled with sealant about the periphery of thewellbore 16 to form an impervious interface in a single step withoutrequiring reactive working fluid or further treatment. Accordingly, itwill be realized that geological permeability does not present anygeothermal heat mining issues considering the scope of the methodsdiscussed herein.

As ancillary benefit, the technology can extend into the remediation andretrofit realms. One of the foundations of the technology herein is anenvironmentally friendly solution to energy creation in the geothermalfield which circumvents fracturing necessitating unappealing fluidhandling. A second foundation is that the technology provides a trulyclosed loop system as opposed to that which has been improperlycharacterized in the prior art as closed loop.

Since the technology allows for a highly effective sealing protocol withthe enumerated geothermal recovery benefits, it has been found that thetechnology can be applied to remediating ineffective, unused orotherwise inoperable geothermal wells. These wells may be unusable orinoperable due to any number of issues such as low flow rates,ineffective fractures, unconsolidated formations and consequent sandproduction problems, excessive corrosion from the brine, or due leachingproblems, among others. Accordingly, where retrofit to a new non tackinggeothermal arrangement as discussed herein is not possible, theinoperable site will be abandoned by removal of unnecessary casing andancillary components, or where possible remediated with expensive andenvironmentally contentious operations such as fracking or potentiallyby redrilling entire wells. In light of the fact that permeabilitydegree is not an issue, the sealing technology presents an attractiveremediation benefit.

Regarding conversion or retrofit of existing geothermal wells, thepre-existence of the wells, allows for the technology to be deployedwith significant economic advantages, eliminates the need for fracturingfluid management, induced seismicity, and environmental risks, andrenders a retrofit site, previously widely recognized as anenvironmental unfriendly energy source, as a green energy platform fromwhich to further build.

In terms of additional implementations of the technology scope,reference will now be made to the advancing figures.

In FIG. 19, the well loop 64 comprises a closed loop system having aninlet well 10 and an outlet well 20, disposed within a geologicalformation, which may be, for example, a geothermal formation, lowpermeability formation, sedimentary formation, volcanic formation or“basement' formation which is more appropriately described ascrystalline rock occurring beneath the sedimentary basin (none beingshown).

The well loop 64 and power cycle 66 are in thermal contact by heatexchanger 68 which recovers heat from the working fluid circulating inthe loop 64 in the formation which is subsequently used to generatepower with generator 70 in cycle 66. As an example, the temperature ofthe formation may be in the range of between 80° C. and 150° C.

In the arrangement illustrated, two distinct working fluids are used.Further detail concerning the fluids will be discussed herein after.Modifying the working fluid used within the well loop operation of thesystem is possible at low temperatures.

As such, currently available power generation modules usually limit theinput temperature of the power cycle working fluid to above 0° C. in theprimary heat exchanger. A higher turbine pressure ratio is enabled bydropping the working fluid temperature below zero. However, conventionalgeothermal projects are limited by potential freezing of the geothermalfluid on the other side of the heat exchanger.

These limitations in present technology are traversed by implementing asegregated power cycle system in combination with a closed loop well.

The fluids may be modified with additives to prevent freezing at subzero° C. temperatures. Suitable additives include, anti-scaling agents,anti-corrosion agents, friction reducers, and anti-freezing chemicals,refrigerants, biocides, hydrocarbons, alcohols, organic fluids andcombinations thereof.

A substantial benefit of the tailored well-loop working fluid incombination with the segregated power cycle is that it is unaffected byvery cold ambient temperatures and thus facilitates use of any genericpower cycle (including ORC, Kalina, carbon carrier cycle, CTPC) to beused to increase higher net power production when used in conjunctionwith a well loop as set forth in FIG. 19. In this arrangement heat istransferred from the first working fluid to the second working fluidwhen the temperature of the second working fluid is at zero ° C. orsubzero ° C.

Optional arrangements with the segregated circuit are illustrated inFIGS. 20 and 21.

FIG. 20 illustrates a segregated circuit incorporating a well loop 12 inthermal contact with two distinct heat exchangers 18 each with its ownpower generator 22 forming a parallel arrangement. Similarly, FIG. 21,illustrates a serial arrangement.

The integrated well loop power cycle is a closed loop system in whichthe selected working fluid is circulated within the well loop and thenflows into a turbine on surface as shown in FIG. 22. Numeral 72 denotesthe overall process schematic. In this process, a single-fluid is usedrather than having a discreet well loop fluid and a secondary powercycle working fluid. The working fluid in this closed loop cycle canoperate either as a transcritical cycle, whereby the fluid issupercritical at the upper working pressure and subcritical at the lowerworking pressure, or as an entirely supercritical cycle whereby thefluid remains supercritical at the lower working pressure.

As is known, a transcritical cycle is a thermodynamic cycle where theworking fluid goes through both the subcritical and supercriticalstates. The apparatus further includes a cooling device, shown in theexample a: 5 an aerial cooler 74 and turbine 76 with generator 78. Theaerial cooler 74 is used to cool he working fluid to a temperaturebetween 1° C. and 15° C. above ambient temperature. It is also to benoted that the working fluid can be cooled to a subzero ° C.temperature. Reference to FIG. 24 delineates performance data.

The driving mechanism in this integrated cycle is a very strongthermosiphon which arises due to the density difference between theinlet vertical well 10 and the outlet vertical well 20. The fluid is ina supercritical liquid state in the inlet well 10, heats up as ittravels along the lateral interconnecting sections 80 and exits in asupercritical state in the outlet well 20, which creates significantpressure.

The thermosiphon effect can completely eliminate the need for a surfacepump under normal operating conditions except during start-up.Advantageously, this eliminates the power required to operate the pumpand increase the net electrical power output.

Working in concert with the well loop circuit is the use of customizedfluids and mixtures tailored to the wellbore layout, depth, length, andambient temperature. The prior art only discusses the use of carbondioxide or pure hydrocarbon fluids. With a closed-loop system such asthat discussed herein, the initial cost and complexity of a fluidmixtures is only a minor factor in the overall economics. So otherfluids can be used such as a fluid having at least one property selectedfrom the group comprising:

-   a working fluid for use in recovering thermal energy from a    geothermal well having an inlet well, an outlet well and an    interconnecting well segment therebetween, said working fluid has at    least one property selected from the group comprising:-   a) a substantially nonlinear temperature enthalpy relationship    within the interconnecting segment at pressures greater than 10 MPa    and temperatures less than 180° C. to maximize the temperature    differential arid heat transfer between the fluid and the    surrounding downhole heat source;-   b) capable of undergoing a pressure-sensitive reversible reaction    which is endothermic at elevated pressure and exothermic at pressure    lower than the elevated pressure;-   c) a fluid mixture containing a chemical absorption reaction which    is endothermic within the interconnecting/lateral section;-   d) an aqueous electrolyte solution with temperature and pressure    dependent solubility, resulting in an endothermic effect within the    interconnecting/ lateral section;-   e) water-based fluid containing a turbulent drag friction reducing    composition that does not degrade when exposed to high shear;-   f) supercritical fluid;-   g) ammonia-ethane mixture;-   h) functional combinations of a) through g)

It has been found that fluids that exhibit a substantially non-lineartemperature-enthalpy relationship within the lateral portion of the wellloop and/or that exhibit a pressure-sensitive reversible reaction whichis endothermic at elevated pressure and exothermic at pressure lowerthan the elevated pressure can increase power generation considerably.This develops because the average temperature differential between thefar-field rock temperature and the circulating fluid temperature isincreased, driving increased heat transfer from the geologic formation.

An example of this type of fluid for use in a segregated configurationis an aqueous precipitate/electrolyte solution withtemperature-dependent solubility, wherein the water is super saturatedat the top of the inlet well. The solid particles are held in suspensionwith an anti-scaling agent (anti-flocculation agent) and with turbulentflow (similar to a drilling mud). In the lateral sections, thetemperature is increasing, hence the solubility of the solids held insuspension is also increasing. This allows the solution toendothermically absorb heat from the rock (basically increases theeffective heat capacity of the fluid) as the solid particles dissolveinto the water. In the heat exchanger to the segregated heat-to-powercycle, temperature is decreasing, so the solid substance isprecipitating exothermically.

Useful fluids include aqueous solutions with the following solutes asexamples: Ammonium acetate,ammonium dihydrogen phosphate,ammoniumformate, ammonium nitrate, potassium bromide, potassium chloride,potassium formate, potassium hydrogen carbonate, potassium nitrate,sodium acetate, sodium carbonate and monosodium phosphate.

To use a single turbine and have adequate efficiency over an entirerange of ambient conditions is problematic. It has been found that useof two or more turbines in series or parallel which are optimized fordifferent ambient conditions addresses the problem. During periods ofcolder temperatures, control logic (not shown) automatically shifts theworking fluid to the appropriate turbine to maintain high efficiencythroughout the year.

Referring now to FIGS. 25 and 25A, shown is a schematic illustration ofa daisy chain of wells, globally denoted by numeral 82. In thisembodiment, each surface location, generally denoted by numeral 84,includes an injection well 86 connected to a lateral well conduit orinterconnecting segment 88 and production well 90. In this manner, thecontinuous well structure subscribes to a generally U shaped structure.The lateral well segment may comprise a well system 36, as discussed inFIG. 3 or any of the other configurations discussed previously.

As illustrated , each location 84 is discrete and linked to proximallocations in an elegant and advantageous manner. As an example, thedistance between locations may be 3,500 meters to 6000 meters. Thiswill, of course, vary from one situation to another.

In operation, working fluid is circulated in the injection well 86 ofone location 84, optionally processed through, for example, a powergeneration apparatus (not shown) to recover the heat energy andsubsequently passed as an output stream to be an inlet feed stream for ainjection well 86 of a proximal location 84. The chain line 92illustrates this relay or daisy chain sequencing. Since not all of theheat is recovered, the inlet feed stream for well 86 of a proximallocation is preheated for injection into lateral conduit 88. The processthen resets for repetition in the next location 84.

Turning now to FIG. 26, shown is a further embodiment of the inventionfor example, a 8,000 kW to 12,000 kW system. In this example, individualloops may be joined at a centralized location 94 in order to centralizethe power generation apparatus (not shown) for increased power andefficiency.

FIGS. 27 and 28 illustrate smaller scale operations, 4,000 kW-6,000 kW(FIGS. 27) and 2,000 kW-3,000 kW (FIG. 26).

One of the significant features of employing the daisy chainimplementation is the lack of a requirement for a near surface returnconduit. When required, as in conventional well loop arrangements,capital costs exceed 10% of the total project capital, there may be aneed to negotiate rights of way and a ˜3° C. heat loss and a pressureloss results causing lower efficiency.

By contrast, the daisy chaining, since well loops are linked front toback, eliminates the need for a near surface return conduit. Further,the paired loops act as the return conduit for each other with the pairusing waste heat as an input to create the preheated stream supra.

Other advantages include increased power production with no surfacedisruption (footprint) since everything is subsurface and reduceddistance between locations 84. This commensurately reduces cost ifshorter conduit 88 can be used owing to the increased temperature of thepreheated feed stream design.

The wells in the examples are formed by employing the sealing whiledrilling methodology described. It will be understood that anycombination of well configurations can be incorporated in the daisy typearrangement. Further, any combination of destructive techniques may beused to form the wellbbores in the daisy example as well as any of theother configurations shown in all of the Figures. In some Figures,reference to an “n” designation is included together with the Figurenumber. An example is FIG. 6 having an area denoted 6 n. This is torepresent that any number, thus n, of additional wells may be stackedvertically or positioned in a parallel relationship or both with the oneshown. The well type may be different or the same for the additionalwells.

As enumerated by the examples, the technology set forth herein ispredicated on the ability to form a geothermal formation, regardless ofpermeability, into an energy efficient wellbore for maximumconductivity. This capability, when coupled with highly effectiveworking fluids, results in remarkable methodology.

Fluid circulation within the wells can occur in any pattern anddirection that facilitates efficiency. This will be partly dictated bythe nature of the formation and will determined and appreciated by thoseskilled in the art.

1. A method for drilling a wellbore into a formation suitable forgeothermal heat recovery, comprising: inducing irreversible formationdamage to said wellbore while drilling said wellbore using at least oneof a thermal mechanism, mechanical mechanism, chemical mechanism andbiological mechanism; and forming an interface between said wellbore andsaid formation substantially impermeable to fluids.
 2. The method as setforth in claim 1, wherein said wellbore is a closed loop, continuouscircuit with said interface extending at least between an inlet well andan outlet well of said loop.
 3. The method as set forth in claim 1,wherein said mechanism is a chemical mechanism.
 4. The method as setforth in claim 3, wherein said chemical mechanism comprises utilizing analkali silicate based drilling fluid.
 5. The method as set forth inclaim 4, wherein said alkali silicate based drilling fluid includes atleast one of potassium, sodium and sodium aluminosilicate.
 6. The methodas set forth in claim 4, wherein said drilling fluid contains between0.3% and 9% by mass in water.
 7. The method as set forth in claim E,wherein said drilling fluid contains between 3% and 6% by mass in water.8. The method as set forth in claim 6, wherein said drilling fluid has apH not less than 10.5.
 9. The method as set forth in claim 1, furtherincluding the step of augmenting the sealing capacity and mechanicalintegrity of a formed interface in a further chemical unit operation.10. The method as set forth in claim 9, wherein said further chemicalunit operation includes treating said interface with at least one ofcalcium chloride brine, acids, CO₂, surfactants and esters.
 11. Themethod as set forth in claim 9, wherein said chemical unit operationcomprises treating said formed interface with a compound capable ofchemically bonding to said formed interface.
 12. The method a set forthin claim 11, wherein said compound comprises exfoliated fly ash.
 13. Themethod a set forth in claim 11, wherein said compound comprises at leastone of surface-activated graphene, graphene oxide, carbon fibres andmixtures thereof.
 14. The method as set forth in any one of claim 1,further including circulating a working fluid within said wellborecontaining an interface maintenance additive for maintainingimpermeability in use
 15. The method as set forth in claim 14, furtherincluding maintaining working fluid pressure within said wellbore at apressure sufficient to maintain structural integrity of said wellborewhen required.
 16. The method as set forth in claim 14, wherein saidworking fluid has at least one property selected from the group,comprising: a) a substantially nonlinear temperature enthalpyrelationship within the interconnecting wellbore section between theinlet well and the outlet well at pressures greater than 10 MPa andtemperatures less than 180° C. to maximize the temperature differentialand heat transfer between said fluid and the surrounding formation; b)capable of undergoing a pressure-sensitive reversible reaction which isendothermic at elevated pressure and exothermic at pressure lower thansaid elevated pressure; c) a fluid mixture containing a chemicalabsorption reaction which is endothermic within said interconnectingwell segment; d) an aqueous electrolyte solution withtemperature-dependent solubility, resulting in an endothermic reactionwithin said connecting section; e) water-based fluid containing aturbulent drag reducing composition that does not degrade when exposedto high shear; f) supercritical fluid; g) ammonia-ethane mixture h)functional combinations of a) through g)
 17. The method as set forth inclaim 16, wherein said supercritical fluid is CO₂.
 18. The method as setforth in claim 9, further including circulating a working fluid withinsaid wellbore containing an interface maintenance additive formaintaining impermeability in use.
 19. The method as set forth in claim14, wherein said interface maintenance additive induces self healing ofany permeability compromised areas of said interface.
 20. The method asset forth in claim 18, wherein said interface maintenance additiveinduces self healing of any permeability compromised areas of saidinterface.
 21. The method as set forth in claim 14, wherein saidinterface maintenance additive precipitates unreacted alkali-silicateremaining from the drilling process.
 22. A method for forming a wellwith an inlet well and an outlet well in a formation suitable forgeothermal heat reccvery, comprising: inducing irreversible formationdamage to a wellbore extending between said inlet well and said outletwell while drilling said wellbore using a chemical mechanism to form aninterface between said wellbore and said formation substantiallyimpermeable to fluids; circulating a chemical composition within saidwellbore capable of inducing precipitate formation with said interfaceto augment the sealing capacity and mechanical integrity of saidinterlace; and circulating a working fluid within the sealed wellborecontaining an interface maintenance additive for maintainingimpermeability during circulation of said working fluid within saidwell.
 23. The method as set forth in claim 22, wherein said well is aclosed loop, continuous circuit with said interface extending at leastbetween an inlet well and an outlet well of said loop.
 24. The method asset forth in claim 23, wherein said working fluid has at least oneproperty selected from the group comprising: a) a substantiallynonlinear temperature enthalpy relationship within an interconnectingwellbore section between said inlet well and said outlet well atpressures greater than 10 MPa and temperatures less than 180° C. tomaximize the temperature differential and heat transfer between saidfluid and the surrounding formation; b) capable of undergoing apressure-sensitive reversible reaction which is endothermic at elevatedpressure and exothermic at pressure lower than said elevated pressure;c) a fluid mixture containing a chemical absorption reaction which isendothermic within said interconnecting well section; d) an aqueouselectrolyte solution with temperature-dependent solubility, resulting inan endothermic reaction within said interconnecting well section; e) awater-based fluid containing a turbulent drag reducing composition thatdoes not degrade when exposed to high shear; f) supercritical fluid; g)ammonia-ethane mixture h) functional combinations of a) through g) 25.The method as set forth in claim 24, wherein said supercritical fluid isCO₂.
 26. The method as set forth in claim 1, wherein said wellborecomprises at least one of a closed loop U shaped well with a spacedapart inlet well and outlet well and lateral well interconnecting saidinlet well and said outlet well, L shaped well with a closed terminalend, tube in tube well, arrangement, grouped closed loop U shaped wellsin spaced relation with an output well member in said group connected toan input well of another group member, a closed loop U shaped wellhaving a plurality of lateral wells commonly connected to a respectiveinlet well and outlet well, a plurality a closed loop U shaped wellhaving a plurality of lateral wells commonly connected to a respectiveinlet well and outlet well arranged with lateral wells of said pluralityarranged with said laterals at least partially interdigitated forthermal contact and combinations thereof.
 27. The method as set forth inclaim 26, further including the step of incorporating a device forstoring, using and/or converting thermal energy from said working fluidcirculating in said closed loop.
 28. The method as set forth in claim23, wherein said wellbore comprises at least one of a closed loop Ushaped well with a spaced apart inlet well and outlet well and lateralwell interconnecting said inlet well and said outlet well, L shaped wellwith a closed terminal end, tube in tube well, arrangement, groupedclosed loop U shaped wells in spaced relation with an output well memberin said group connected to an input well of another group member, aclosed loop U shaped well having a plurality of lateral wells commonlyconnected to a respective inlet well and outlet well, a plurality aclosed loop U shaped well having a plurality of lateral wells commonlyconnected to a respective inlet well and outlet well arranged withlateral wells of said plurality arranged with said laterals at leastpartially interdigitated for thermal contact and combinations thereof.29. The method as set forth in claim 28, further including the step ofincorporating a device for storing, using and/or converting thermalenergy from said working fluid circulating in said closed loop.
 30. Amethod for remediating a well including at least one of fracturedsections created by fracturing techniques, unconsolidated rock and sandwithin an earth formation, comprising: treating said well and pore spaceof said at least one of fractured sections, unconsolidated rock and sandwith a first chemical composition capable of forming a precipitatedimpervious interface at said sections; and treating said interface witha second chemical composition for precipitating any unreacted firstchemical composition to further seal said interface.
 31. The method asset forth in claim 30, wherein said first chemical composition is analkali silicate fluid.
 32. The method as set forth in claim 31, whereinsaid alkali silicate fluid includes at least one of potassium, sodiumand sodium aluminosilicate.
 33. The method as set forth in claim 30,wherein said second chemical composition includes at least one ofcalcium chloride brine, acids, CO₂, surfactants and esters.
 34. A methodfor converting a fracturing based geothermal well having at least one offractures, unconsolidated rock and sand, an inlet well and an outletwell in fluid communication, to a closed loop geothermal well,comprising: introducing a first chemical composition capable of forminga precipitated impervious interface between said inlet well and saidoutlet well and in said at least one of fractures, unconsolidated rockand sand whereby a closed sealed loop is formed with said at least oneof fractures, unconsolidated rock and sand, inlet well, the outlet well,and the area there between; and treating said interface with a secondchemical composition for precipitating any unreacted first chemicalcomposition to further seal said interface.
 35. The method as set forthin claim 34, further including the step of circulating a working fluidwithin said closed loop capable of reacting with said interface toinduce a precipitation reaction at said interface.
 36. The method as setforth in claim 34, further including the step of continuouslycirculating said working fluid within said closed loop.
 37. The methodas set forth in claim 36, further including the step of incorporating adevice for storing, using and/or converting thermal energy from saidworking fluid 8 circulating in said closed loop.
 38. The method as setforth in claim 34, optionally including drilling from said inlet well tosaid outlet well to form a wellbore loop continuous from said inlet wellto said outlet well.
 39. A method of forming a geothermal well having aninlet well and an outlet well, comprising: providing a drilling fluidcapable of sealing an open hole wellbore between said inlet well andsaid outlet well; and sealing while drilling said open hole wellbore toform an impervious interface between the interior of the wellbore andthe surrounding formation.
 40. The method a set forth in claim 39,further including inducing a second sealing operation at said interface.41. The method a set forth in claim 39, further including inducing athird sealing operation at said interface.
 42. The method a set forth inclaim 41, wherein said third sealing operation occurs dynamically duringfluid circulation within said well.
 43. A method of forming a wellwithin a geothermal formation for energy recovery, comprising: drillingan open hole wellbore into a geothermal formation; introducing reactivechemical compositions into said wellbore for reaction to form a fluidimpervious interface between said wellbore and said formation, saidinterface including unreacted reactive chemical compositions; andintroducing a working fluid into said wellbore capable of reacting withsaid unreacted reactive chemical compositions for further formation ofsaid interface.
 44. The method as set forth in claim 43, wherein saidunreacted reactive chemical compositions are intermediate between saidinterface and said formation.
 45. The method as set forth in claim 43,wherein said well is a closed loop, continuous circuit with saidinterface extending at least between an inlet well and an outlet well ofsaid loop.
 46. The method as set forth in claim 45, wherein said workingfluid is circulated within said closed loop for capturing thermal energyfrom within said formation and maintaining seal integrity throughreaction with unreacted reactive chemical compositions of saidinterface.
 47. The method as set forth in claim 46, wherein said workingfluid is circulated within said loop in a variable manner.
 48. Themethod as set forth in claim 47, wherein said variable manner includesperiods of quiescence.